Transcript Chapter 10x

Chapter 10
The Interstellar Medium
Guidepost
In a discussion of bread baking, we might begin with a
chapter on wheat and flour. In our discussion of the birth
and death of stars, the theme of the next five chapters, we
begin with a chapter about the gas and dust between the
stars. It is the flour from which nature bakes stars.
This chapter clearly illustrates how astronomers use the
interaction of light and matter to learn about nature on the
astronomical scale. That tool, which we developed in
Chapter 7, “Starlight and Atoms,” is powerfully employed
here, especially when we include observations at many
different wavelengths.
We also see in this chapter the interplay of observation
and theory. Neither is useful alone, but together they are a
powerful method for studying nature, a method generally
known as science.
Outline
I. Visible-Wavelength Observations
A. Nebulae
B. Extinction and Reddening
C. Interstellar Absorption Lines
II. Long- and Short-Wavelength Observations
A. 21-cm Observations
B. Molecules in Space
C. Infrared Radiation from Dust
D. X Rays From the Interstellar Medium
E. Ultraviolet Observations of the Interstellar
Medium
III. A Model of the Interstellar Medium
A. Four Components of the Interstellar Medium
B. The Interstellar Cycle
A World of Dust
The space between the stars is not
completely empty, but filled with very
dilute gas and dust, producing some of
the most beautiful objects in the sky.
We are interested in the interstellar
medium because
a) dense interstellar clouds are the
birth place of stars
b) Dark clouds alter and absorb the
light from stars behind them
Bare-Eye Nebula: Orion
One example of an
interstellar gas cloud
(nebula) is visible to the
bare eye: the Orion nebula
Three Kinds of Nebulae (1)
• Hot star
illuminates a
gas cloud…
1) Emission Nebulae
• excites and/or
ionizes the gas
(electrons kicked
into higher
energy states)…
• electrons
recombining,
falling back to
ground state
produce
emission lines.
The Fox Fur Nebula
The Trifid
NGC 2246Nebula
Three Kinds of Nebulae (2)
• Star illuminates gas and
dust cloud;
• star light is reflected
by the dust;
• reflection nebula appear
blue because blue light
is scattered by larger
angles than red light;
• Same phenomenon
makes the day sky
appear blue (if it’s not
cloudy).
2) Reflection Nebulae
Scattering in Earth’s Atmosphere
(SLIDESHOW MODE ONLY)
Three Kinds of Nebulae (3)
Dense clouds of gas and dust absorb the light
from the stars behind;
3) Dark Nebulae
appear dark
in front of
the brighter
background;
Bernard 86
Horsehead Nebula
Interstellar Reddening
Blue light is strongly scattered and
absorbed by interstellar clouds
Red light can more
easily penetrate the
cloud, but is still
absorbed to some
extent
Infrared radiation
is hardly
absorbed at all
Barnard 68
Visible
Interstellar
clouds make
background
stars appear
Infrared redder
Interstellar Reddening (2)
The Interstellar Medium absorbs light
more strongly at shorter wavelengths.
Interstellar Absorption Lines
These can be
distinguished from
stellar absorption
lines through:
The interstellar medium
produces absorption lines in
the spectra of stars.
a) Absorption from
wrong ionization
states
Narrow absorption lines from Ca II: Too low
b) Small line width
ionization state and too narrow for the O
(too low
star in the background; multiple components
temperature; too
low density)
c) Multiple
components
(several clouds of
ISM with different
radial velocities)
Structure of the ISM
The ISM occurs in two main types of clouds:
• HI clouds:
Cold (T ~ 100 K) clouds of neutral hydrogen (HI);
moderate density (n ~ 10 – a few hundred atoms/cm3);
size: ~ 100 pc
• Hot intercloud medium:
Hot (T ~ a few 1000 K), ionized hydrogen (HII);
low density (n ~ 0.1 atom/cm3);
gas can remain ionized because of very low density.
Observing Neutral Hydrogen:
The 21-cm (radio) line (I)
Electrons in the ground state of neutral hydrogen have
slightly different energies, depending on their spin
orientation.
Opposite magnetic
fields attract =>
Lower energy
Magnetic field
due to proton spin
21 cm line
Magnetic field
due to electron
spin
Equal magnetic
fields repel =>
Higher energy
The 21-cm Line of Neutral Hydrogen (II)
Transitions from the higher-energy to the lowerenergy spin state produce a characteristic 21-cm
radio emission line.
=> Neutral
hydrogen
(HI) can be
traced by
observing
this radio
emission.
Observations of the 21-cm Line (1)
G a l a c t i c
p l a n e
All-sky map of emission in the 21-cm line
Observations of the 21-cm Line (2)
HI clouds moving towards Earth
HI clouds moving
away from Earth
Individual HI clouds
with different radial
velocities resolved
(from redshift/blueshift of line)
Molecules in Space
In addition to atoms and ions, the interstellar
medium also contains molecules.
Molecules also store specific energies in their
a) rotation
b) vibration
Transitions between different rotational /
vibrational energy levels lead to emission
– typically at radio wavelengths.
The Most Easily Observed
Molecules in Space
• CO = Carbon Monoxide  Radio emission
• OH = Hydroxyl  Radio emission.
The Most Common Molecule
in Space:
• H2 = Molecular Hydrogen  Ultraviolet
absorption and emission:
Difficult to observe!
But: Where there’s H2, there’s also CO.
Use CO as a tracer for H2 in the ISM!
Molecular Clouds
• Molecules are easily destroyed
(“dissociated”) by ultraviolet photons
from hot stars.
 They can only survive within dense, dusty clouds,
where UV radiation is completely absorbed.
 “Molecular Clouds”:
UV emission from
Molecules
nearby stars destroys
survive
molecules in the outer
parts of the cloud; is Cold, dense
molecular
absorbed there.
cloud core
Diameter ≈ 15 – 60 pc
HI Cloud
Temperature ≈ 10 K
Largest molecular
clouds are called
“Giant Molecular
Clouds”:
Total mass ≈ 100 – 1 million solar masses
Interstellar Dust
Probably formed in
the atmospheres of
cool stars.
Mostly observable
through infrared
emission.
Infrared and radio
emissions from
molecules and
dust are efficiently
cooling gas in
molecular clouds
IRAS (infrared) image
of infrared cirrus of
interstellar dust.
The Coronal Gas
Additional component
of very hot, low-density
gas in the ISM:
X-ray image of the
Cygnus region
T ~ 1 million K
n ~ 0.001 particles/cm3
Observable in X-rays
Called “Coronal gas”
because of its
properties similar to
the solar corona (but
completely different
origin!)
Our sun is located within
Probably originates in supernova explosions (near the edge of) a
coronal gas bubble.
and winds from hot stars
The Four Components of the
Interstellar Medium
Component
Temperature
[K]
Density
[atoms/cm3]
Main
Constituents
HI Clouds
50 – 150
1 – 1000
Neutral
hydrogen; other
atoms ionized
Intercloud Medium
(HII)
103 - 104
0.01
Partially ionized
H; other atoms
fully ionized
Coronal Gas
105 - 106
10-4 – 10-3
All atoms highly
ionized H
Molecular Clouds
20 - 50
103 - 105
Neutral gas;
dust and
molecules
The Interstellar Cycle
Stars, gas, and dust are in constant interaction with each other.
Stars are formed from dense
molecular cloud cores.
Hot stars
ionize gas,
producing
HII regions.
Young star clusters
illuminate the remnants
of their “mother”
clouds, producing
reflection nebulae
Supernovae
trigger shock
waves in the ISM
that lead to the
compression of
dense clouds and
new star
formation.
Supernovae of
Young star clusters leave
trails of rarefied ISM behind. massive stars
produce coronal
gas and enrich
the ISM with
heavier elements.
New Terms
interstellar medium
nebula
emission nebula
HII region
reflection nebula
dark nebula
forbidden line
metastable level
interstellar dust
interstellar extinction
interstellar reddening
interstellar absorption
lines
HI clouds
intercloud medium
pressure
21-cm radiation
molecular cloud
giant molecular clouds
infrared cirrus
coronal gas
local bubble or void